The present invention relates to non-contact optical sensors, and more particularly non-imaging interferometric devices for characterizing objects, in terms of determining time dependent and time independent dimensions.
Devices capable of measuring the cross sectional dimensions of narrow, elongate objects are useful in many scientific and technological applications. For example, in the manufacture of fibers (including optical fibers) and wires, typically by drawing processes, sensors are employed to monitor fiber and wire diameters.
Several types of these sensors are presently used in the fiber optics industry. In one approach, detector arrays image the fiber. In another, a focused laser beam scans the fiber. Both techniques have relatively slow response times, in that a measurement typically requires at least one tenth of a second. Given the high speed at which fibers are moved in many drawing processes (e.g. 10-100 meters per second), the diameter measurement must be averaged over several meters of fiber length. As a result, defects in the optical fibers are sensed only if they are sufficiently severe to substantially alter the fiber dimension, or if they occur over a considerable length of the fiber. Less severe, localized defects are not detected.
According to a third known technique, coherent light is directed onto the fiber in a manner that causes the fiber to produce a diffraction pattern. This approach affords more rapid measurement to facilitate detection of localized defects. However, its sensitivity is low. For example, an optical fiber having a diameter of 125 microns typically can be measured with a resolution of about 1 micron, at best.
Laser phase Doppler systems are known for their utility in measuring instantaneous velocities and diameters of fine particles in two-phase flows, e.g. liquid sprays. In a conventional phase Doppler apparatus, two spatially separated laser beams are caused to intersect one another, where they interfere with one another to create a measuring volume. The measuring volume is so small that usually only one particle crosses it at a time. Several photodetectors receive light scattered by particles passing through the measuring volume. The photodetectors are spaced apart from one another in a scattering plane, typically perpendicular to the plane formed by the laser beams. Differences in phase can be used to determine particle size, while heterodyne Doppler frequency is employed to determine particle velocity.
Particles, especially spherical particles, tend to scatter light in all directions and thus lend themselves well to analysis by laser Doppler systems. Elongate cylinders, by contrast, tend to scatter light over more restricted regions. More particularly, a cylinder arranged with its longitudinal axis perpendicular to the plane defined by the pair of laser beams tends to scatter light in the form of a thin sheet lying within the beam plane. Thus, conventionally arranged photodetectors are not well positioned for characterizing the cylinder.
As disclosed in the aforementioned application, high speed and accuracy are achieved by detecting, at two different locations, laser energy scattered by the fiber. However, certain manufacturing processes and conditions present challenges in this regard. For example, the phase difference (phase shift) measurement depends upon the fiber refractive index as well as the fiber diameter. The most common cause for a change in refractive index is a change in temperature, although the refractive index also is influenced by a change in material composition. A change in refractive index, if undetected, can lead to an erroneous conclusion as to fiber diameter.
Therefore, it is an object of the present invention to provide a fiber diameter measurement system incorporating correction for changes in the fiber refractive index and enhanced measurement accuracy through combining two separate measurements of object diameter.
A further object is to provide a system for combining phase shift readings to more accurately measure cross sectional dimensions of cylindrical objects.
Yet another object is to provide a means for measuring fibers, wires and other elongated cylindrical members as they are being manufactured, in a manner that avoids contact with the objects yet provides accurate and real time information to substantially improve quality control.